PROJECT SUMMARY Musculoskeletal trauma is highly prevalent in both combat-wounded and civilian patients, and despite advances in trauma care, mortality and complication rates remain remarkably high. Patients who do not respond to treatment or suffer from complications, experience poor healing, longer hospital stays, increased treatment costs, and prolonged disability. Recently, systemic immune dysregulation and immunosuppression has been implicated in the limited success of current intervention strategies and poor outcomes in trauma patients. Systemic immune dysregulation and immunosuppression results in decreased levels of circulating pro-inflammatory cytokines and a decrease in circulating immune effector cells, such as effector T cells. Another notable hallmark of systemic immune dysregulation is elevated levels of immune suppressor cells, including myeloid-derived suppressor cells (MDSCs). Our overall objectives are to investigate (i) how the development of systemic immune dysregulation and immunosuppression relates to functional bone regeneration and (ii) how systemic immunomodulation impacts the immune system and regenerative outcomes following severe musculoskeletal trauma. We will meet these objectives through the following specific aims. SPECIFIC AIM 1: Characterize the development of systemic immune dysregulation and immunosuppression in a pre-clinical composite trauma model and identify immunological markers predictive of poor healing. Using a composite bone/muscle trauma pre- clinical model, we will longitudinally characterize systemic immune dysregulation biomarkers via flow cytometry and multi-analyte serum proteomic profiling and then utilize nonlinear evolutionary multivariate analytics to develop predictive models of functional bone regeneration. SPECIFIC AIM 2: Fabricate and optimize an immunomodulatory therapeutic to target and deplete MDSCs. We will develop synthetic gold nanoparticles functionalized with both Fc-mimicking and MDSC-targeting ligands that can mimic mAb function (SNAbs) and optimize their multivalency, Janus orientation, and size for MDSC depletion. SPECIFIC AIM 3: Evaluate the effect of systemic immunomodulation on the immune system status and bone regeneration in vivo following severe musculoskeletal trauma. We will test the hypothesis that targeted depletion of MDSCs will restore immune homeostasis and promote functional bone regeneration.